[turbofan] Remove unnecessary line.

I had forgotten to upload this to my previous CL.

TBR: mslekova@chromium.org
Change-Id: I1195ffd947ad82226af41cd7103b389a6733f4e4
Reviewed-on: https://chromium-review.googlesource.com/c/1288590Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56766}

src/compiler/js-heap-broker.h

@@ -121,8 +121,6 @@ class ObjectRef {
 
  private:
   JSHeapBroker* broker_;
-
- protected:
 };
 
 // Temporary class that carries information from a Map. We'd like to remove